1. Field of the Invention
The present invention concerns an optical device for X-ray instrumentation applications with high resolutions in wavelength.
More precisely, the invention concerns an optical device intended to treat an incident X-ray beam, said device comprising:                a monochromator, and        an optical element for treating the incident beam whose reflective surface is able to produce a two-dimensional optical effect in order to adapt a beam in destination of the monochromator, said optical element comprising a surface reflecting X-rays of the multilayer structure type.        
The reflective surface or surfaces used can in particular be of the multilayer type with lateral gradient.
The invention thus applies in all X-ray instrumentation fields using monochromators.
By way of example, the following applications can be cited non-limitingly:                high-resolution X-ray diffractometry,        X-ray fluorescence,        X-ray micromapping (or microcartography) applications for microelectronics.        
The invention applies to X-ray instrumentation fields requiring excellent spectral purity and therefore the use of a monochromator.
The basic constituent element of the monochromator is a crystal which makes it possible to achieve very high resolutions, angular and in terms of wavelength. The monochromator can be formed from a crystal or several aligned crystals.
For monochromators of the type mentioned above, the diffraction of the incident X-rays is effected according to Bragg's law.
The Bragg condition for a crystal is of the form nλ=2d sin θβ where n is the reflection order, λ the wavelength of the incident radiation for which the diffraction occurs, d the spacing period between the atomic planes of the crystal involved in the diffraction and θβ the angle of incidence on these same atomic planes which is necessary for the diffraction phenomenon to occur.
If an incident beam of X-rays is considered, the rays of wavelength λ striking the crystal with an angle of incidence θβ which is very precise with respect to a certain family of atomic planes of the crystal will be diffracted by these same atomic planes if the Bragg condition indicated above is satisfied.
This phenomenon of diffraction of a monochromatic beam occurs with a certain angular acceptance Δθ about the reference angle θβ.
This angular acceptance can therefore be defined by:                An angle θβ corresponding to the reference angle of incidence of the diffractive rays on the monochromator (θβ is known by the term Bragg angle), θβ being a function of the crystal and the wavelength and corresponding to the maximum of the reflectivity peak R=f(θ) for a given wavelength, and        A tolerance of Δθ about this reference angle of incidence. The tolerance defines the width of the range of angles of incidence which corresponds to the angular acceptance.        
The monochromators used in the devices of the type mentioned above have a very small angular acceptance. By way of example, for a germanium crystal monochromator, used for example for applications where the X-ray source is a Kα copper source (λ=1.54 Angstroms), the angular acceptance is 0.00336° (about a reference angle of incidence of approximately 20°).
It will therefore be understood that, from a given X-ray source (this source being able for example to be of the rotating anode, X-ray tube or microsource type), without an appropriate conditioning of the X-rays emitted by the source, a large number of these rays which are emitted in all directions arrive at the monochromator with an angle of incidence well outside the angular acceptance of the monochromator.
These photons will not be able to be reflected by the monochromator and thus cause very large losses of flux.
2. Description of Related Art
To attempt to mitigate this drawback, it is known to dispose, upstream of the monochromator, means of conditioning the incident beam.
The main function of such conditioning means is to orient the largest possible part of the incident X-rays, at an angle of incidence (with respect to the surface of the monochromator) which is included within the incidence range defined by the angular acceptance of the monochromator about a reference angle of incidence θβ.
It is thus known to produce these conditioning means in the form of a glass capillary for collecting by total reflection a divergent initial beam issuing from a source and to collimate it into a beam directed towards a monochromator.
However, one limitation associated with such conditioning means is that this type of optical component can reflect X-rays only at very small angles of incidence (typically less than 0.1°).
Consequently the flux delivered by the optics is generally small.
It also known to produce the conditioning means in the form of a multilayer optical element producing a one-dimensional optical effect. These optical elements have a parabolic shape which makes it possible to collimate the divergent incident beam, and a multilayer coating which diffracts the incident X-rays according to Bragg's law.
One illustration of this known configuration will be found in FIG. 1, which depicts a source S of X-rays producing an initial beam X1 having a certain divergence in destination of conditioning means 31 (the parabola in which the surface of these conditioning means fits being depicted in a broken line).
Here also, the conditioning means reflect the initial beam X1 as a beam X2 directed towards a monochromator M.
A one-dimensional optical element of this type is known by the term Göbel mirror.
In the case of curved substrates such as Göbel mirrors, the multilayer has a layer structure (meaning thereby the period d of the multilayer) which varies along the mirror in order to maintain the Bragg conditions on a large surface of the mirror.
Such a multilayer mirror with lateral gradient thus allows reflection of the X-rays whose wavelength belongs to a predetermined domain, by different regions of the mirror on which the incident rays have variable local angles of incidence.
Such conditioning means make it possible to collimate the incident beam into a beam X2 in which the directions of propagation of the X-rays are made substantially parallel to an incident direction with respect to the monochromator which corresponds to the value θβ of this monochromator, and this within the angular acceptance range of the monochromator.
However, such conditioning means allow the collimation of an initial beam X1 only in a single plane (the plane of FIG. 1 in the example which has just been described).
The divergences in the planes perpendicular to this plane are thus not treated: as a result many X-rays are not usable.
One limitation of these known conditioning means with one-dimensional effect is thus that, for a given initial beam X1, the flux of collimated X-rays in a direction compatible with the angular acceptance of the monochromator remains limited.
It should also be stated in this regard that it is necessary to have at the output from the monochromator a beam of small size in the fields of application of the invention (typically less than 2 mm).
The beam issuing from the monochromator in fact generates an “image spot” whose dimensions must be of this order of magnitude.
The image spot is included in a plane known as the “image plane”.
To increase the “useful” flux arriving at the monochromator, it is known how to produce the means of conditioning the initial beam in the form of two-dimensional optics whose reflective surface exhibits a lateral gradient.
Such optics are produced in the form of a “side by side Kirkpatrick-Baez” device, as illustrated in FIG. 2.
In the remainder of this text, the “Kirkpatrick-Baez” configuration will be referred to as “KB”.
This figure thus illustrates an element 33 comprising two mirrors 331 and 332 associated side by side (axis parallel to the direction Z for the mirror 331, to the direction X for the mirror 332).
The surfaces of these two mirrors have curvatures centred on two axes perpendicular to one another.
For this type of optics, the conditioning desired is provided by a double reflection, each mirror 331, 332 producing a one-dimensional optical effect along one axis.
Each of the two mirrors can thus produce a collimation or a focusing.
A monochromator M receives the flux X2 reflected by the element 33.
A description of this type of optical element 33 will be found in the patent U.S. Pat. No. 6,041,099.
It should be stated that the conditioning means can also be produced in the form of a “KB” device where the two mirrors are not disposed side by side.
Compared with conditioning means of the Göbel mirror type, such conditioning means with two-dimensional effect make it possible to recover, within a range of angles of incidence compatible with the angular acceptance of a monochromator, a greater proportion of rays issuing from a divergent initial beam X1.